Mount Prospect, Illinois 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

525
FXUS63 KLOT 201114
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
614 AM CDT Sat Jun 20 2026

.KEY MESSAGES...

- A soaking rainfall is likely for most or all of the area on
  Sunday. Some potential for thunderstorms and flooding also
  exists, though whether these threats will materialize here
  remains uncertain.

&&

.DISCUSSION...
Issued at 320 AM CDT Sat Jun 20 2026

Through Tonight:

Modest isentropic ascent within a low- to mid-level frontal
zone activated a pocket of modest elevated instability over our
forecast area a little while ago, allowing for a few storms to
develop near and just north of the I-80 corridor. This activity
has since dwindled as it has progressed to the east-southeast,
but with the isentropic upglide expected to persist across our
southern CWA for a while longer, scattered showers and perhaps
a few additional rumbles of thunder may continue to be observed
there through a little after sunrise this morning.

Otherwise though, expect another dry day today with plenty
of sunshine, comfortable humidity levels, and high temperatures
similar to or just a touch warmer than yesterday`s highs. A
lake breeze will also develop once again and keep some of our
northwest Indiana locales slightly cooler relative to elsewhere.
This lake breeze is likely to eventually make a push inland
into northeast Illinois as well, but likely not until the breezy
west-northwesterly synoptic gradient flow there today starts to
relax with the approach of sunset.


Sunday:

An upper-level shortwave trough will eject eastward out of the
central Rockies today, spurring the development of a surface low
beneath it while also inducing widespread convection across the
central Plains this afternoon and evening. The exact track and
strength of this disturbance and associated surface low is of
relevance for our area as this will largely dictate whether a
threat for thunderstorms (including ones that may be severe) and
flooding could materialize in our forecast area on Sunday
amidst 1.25-2" precipitable water values. Generally speaking, a
more northerly track for these features and a deeper surface low
would result in a greater threat for thunderstorms and flooding
in our area, while a more southerly and weaker solution would
tend to keep these threats suppressed to our south.

Compared to 24 hours ago, there does appear to more support in
general across the latest CAM and ensemble suites for a more
southerly solution that keeps the surface low, an attendant
warm front, richer boundary layer moisture, and appreciable
instability south to potentially well south of our forecast
area. With the expectation that at least one southeastward-
propagating MCS will come out of the central Plains convection
tonight, it appears more likely than not that convection will
just continue to regenerate along the associated outflow into
tomorrow morning and afternoon, which would prohibit the
aforementioned warm front from advancing northward much, if at
all, tomorrow. Thus, do believe that these more southerly model
solutions have some credence to them and that the threat for
thunderstorms and the greatest threat for flooding on Sunday
could very well remain to our south. That said, support for
this solution is not unanimous across the broader spectrum of
model guidance, and if the aforementioned morning MCS doesn`t
materialize, decays much quicker than presently expected, or is
delayed by at least a few hours, then that would open the door
for the warm front to surge northward towards our southern CWA
tomorrow.

Regardless of which scenario pans out, it is expected that most
or all of our forecast area will receive a soaking rainfall on
Sunday given the abundance of both forcing support aloft and
moisture through the tropospheric column (our northernmost
counties could still potentially miss out on this soaking
rainfall if one of the more extreme southerly solutions were to
play out, which is not out of the question at this point). There
has also been a consistent model signal for a pronounced
deformation band to develop along the northern flank of the
surface low. How efficiently this band is able to crank out
rainfall will depend on the strength of the surface low and the
associated frontogenesis, but most model guidance suggests that
a corridor of 2-4" rainfall totals will be observed somewhere in
or just south of our forecast area when all is said and done.
With many area creeks, streams, and rivers still a bit more
elevated than usual from our recent rainfall, amounts like
these do raise concern for additional river rises and flooding
and possibly areal flooding in some locations as well, so will
issue a Hydrologic Outlook to give greater awareness to this
possibility.


Monday through Friday:

After Sunday`s system clears the region, quieter and drier
conditions should settle into the area for the beginning of the
upcoming work week. However, there remains a fairly consistent
signal in ensemble and deterministic guidance for another upper-
level disturbance to arrive in the region mid-week and bring a
renewed likelihood for precipitation to our area. Late Tuesday
into Wednesday still appears to be the favored time frame for
precipitation associated with this system to occur in our
forecast area. Thereafter, guidance indicates that there could
be a couple of follow-up shortwaves embedded within the
northwesterly flow aloft that track through the Midwest during
the latter half of the week in this lead disturbance`s wake.
This may result in additional showery periods beyond Wednesday
into the following weekend amidst continued near to below normal
temperatures.

Ogorek

&&

.AVIATION /12Z TAFS THROUGH 18Z SUNDAY/...
Issued at 614 AM CDT Sat Jun 20 2026

Key Messages:

- Breezy westerly winds (gusts 20-25 kt) develop later this
  morning and continue into the afternoon.

- Lake breeze wind shift likely by early (late) this evening
  (afternoon) at MDW/ORD (GYY).

- Easterly winds Sunday with rain developing by midday.

While there are no significant aviation weather concerns
anticipated through the period, breezy westerly winds (gusts
20-25 kt) will develop later this morning into this afternoon.
Then, as the winds begin to abate into early this evening, a
lake breeze is likely to shift the winds onshore at ORD, MDW and
GYY around, or just before sunset. Wind speeds should ease
quickly following the easterly wind shift, with winds likely
going light (under 5 kt) and variable by mid to late evening as
a surface ridge axis shifts overhead.

On Sunday, the morning will start out dry. However, our next
weather system will quickly be approaching from the west. As it
does, expect surface winds to settle from an easterly direction
after daybreak. Rain will also develop across the terminal
airspace late Sunday morning, with periods of a soaking rain and
associated low MVFR to IFR CIGs and VSBYs expected Sunday
afternoon into the evening. Given that the best rain chances and
lower flight categories will come just beyond the current 30
hour ORD and MDW TAFs, we have opted to introduce a VCSH mention
at the end of the period.

KJB

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...None.

&&

$$

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